This application makes reference to, incorporates the same herein and claims all benefits insuring under 35 U.S.C. xc2xa7120 as a continuation-in-part of my application entitled Subscriber Matching Circuit For Electronic Exchange earlier filed in the United States Patent and Trademark Office on Aug. 25, 1999 and there duly assigned Ser. No. 09/382,681.
1. Field of the Invention
The present invention relates to a subscriber matching circuit for an electronic exchange. In particular, the present invention relates to a subscriber matching circuit for an electronic exchange which matches transmitted/received data so as to permit call communications between the exchange and subscribers.
2. Description of the Related Art
Generally, in an electronic exchange, a subscriber matching circuit for matching with subscribers requires the functions of line current supply, supervision of a subscriber""s state, call signal supply (ring signal supply) and detection, 2-line/4-line conversion, etc. The line current supply function serves to supply current for operating a subscriber""s telephone and to supervise the on-hook/off-hook state of the telephone by detecting the variation of the supplied current. The line current supply function, which is performed with a function of limiting a maximum line current, serves to prevent an unnecessary power consumption due to an oversupply of the line current to a short-distance subscriber. The 2-line/4-line conversion function serves to convert a 4-line signal transmitted from the telephone exchange or through transmission lines into a 2-line signal, while converting the 2-line signal transmitted from the subscriber""s telephone into the 4-line signal.
Such an analog type subscriber matching circuit may employ a transformer. However, it cannot be adapted to the present-day trend of the high-density integration and miniaturization due to its large size and magnetic saturation caused by the line current.
In order to adapt the trend of the high-density integration and miniaturization, the subscriber matching circuit has been integrated into an SLIC integrated circuit (IC). A subscriber matching circuit using the SLIC IC is disclosed in Korean Patent Application No.1994-40809 filed by the applicant of the present application. According to the subscriber matching circuit disclosed in Korean Patent Application No.1994-40809, however, all circuit elements are integrated onto one chip, and thus it is relatively vulnerable to external impact such as lightning strikes in comparison to the circuit employing a transformer. As a result, it requires a protection device as well as many peripheral parts thereof for performing the subscriber matching function, thereby increasing the possibility of experiencing difficulties in operation.
The following patents each discloses features in common with the present invention: U.S. Pat. No. 4,887,293 to Molnar, entitled TRUNK CIRCUIT WITH LOOP LENGTH GAIN EQUALIZATION, issued on Dec. 12, 1989; U.S. Pat. No. 4,953,200 to Yamasaki, entitled PRIVATE BRANCH EXCHANGE CAPABLE OF DISCRIMINATING DIFFERENT TYPES OF TELEPHONE SETS CONNECTED THERETO, issued on Aug. 28, 1990; U.S. Pat. No. 4,358,643 to Levy, entitled TWO TO FOUR WIRE HYBRID CIRCUIT, issued on Nov. 9, 1982; U.S. Pat. No. 5,602,912 to Hershbarger, entitled TELEPHONE HYBRID CIRCUIT, issued on Feb. 11, 1997; U.S. Pat. No. 5,608,795 to Gay, entitled TELEPHONE LINE INTERFACE CIRCUIT, issued on Mar. 4,1997; U.S. Pat. No.5,710,811 to Tomasini et al., entitled SPEECH CIRCUIT FOR SUBSCRIBER TELEPHONE APPARATUS, issued on Jan. 20, 1998; U.S. Pat. No. 4,456,991 to Chea Jr. et al., entitled TELEPHONE LINE CIRCUIT AND SYSTEM, issued on Jun. 26, 1984; U.S. Pat. No. 4,381,561 to Treiber, entitled ALL DIGITAL LSI LINE CIRCUIT FOR ANALOG LINES, issued on Apr. 26, 1983; U.S. Pat. No. 4,759,059 to Christensen, entitled ANALOG TELEPHONE CIRCUIT FOR DIGITAL TELEPHONE SYSTEM, issued on Jul. 19,1988; U.S. Pat. No. 3,982,078 to Janssen et al., entitled LINE MATCHING CIRCUIT FOR USE IN A TONE PUSHBUTTON DIALING SUBSCRIBER""S SET PROVIDED WITH A TONE GENERATOR, issued on Sep. 21, 1976; and U.S. Pat. No. 4,461,929 to Britt, entitled AMPLIFIER FOR ELECTRONIC AND ELECTRO-MECHANICAL TRANSMITTERS, issued on Jul. 24, 1984.
It is an object of the-present invention to solve the problems involved in the related art, and to provide a hybrid subscriber matching circuit for an electronic telephone exchange which can simplify the peripheral parts thereof and strongly resist external impact.
In order to achieve the above object, there is provided a subscriber matching circuit for a full electronic exchange comprising: transistors Q1 and Q2 for supplying a line current to a subscriber. through a tip terminal and a ring terminal; transistors Q3 and Q4, having a Darlington structure and respectively connected to the transistors Q1 and Q2, for limiting a maximum current; current supervising resistors R1 and R2, respectively connected to emitters of the transistors Q1 and Q2, for performing a current feedback operation to limit the maximum current and detecting in a voltage from a line current flowing through telephone lines; a resistor R3, connected between a collector of the transistor Q1 and a collector of the transistor Q3, for preventing the transistor Q1 from being saturated; a resistor R4, connected between a collector of the transistor Q2 and a collector of the transistor Q4, for preventing the transistor Q2 from being saturated; three bias resistors R5, R6 and R7 for determining a threshold value of the maximum current and causing the transistors Q1 and Q2 to always be in an active region; capacitors C5 and C6 for superimposing a received AC audio signal on the DC line current; composite impedances ZL1 and ZL2 for matching with a line characteristic impedance; a resistor R11 for converting the line current flowing through the resistor R1 into an input current for detecting an off-hook state; an operational amplifier AMP3 for inversion-amplifying a signal inputted through the resistor R1, and a transistor Q5 for converting a level of a signal inversion-amplified by the operational amplifier AMP3.